As with value encoding, the purpose of dictionary encoding is to reduce the number of bits required to store the values in a column. Dictionary encoding starts by building a dictionary table that contains a set of distinct values in a column. It then replaces the original values of a column with an integer number that references the index of the original value in the dictionary table.
The dictionary-encoding process is illustrated in Figure 12-4:
Figure 12-4: The dictionary-encoding process
There are two key advantages of using dictionary encoding, as follows:
- All columns contain only integer values.
- The number of bits ...
